1. Field of the Invention
This invention relates to a filter comprising nonwoven, blown polypropylene microfiber sheets which is especially useful in solid phase extraction processes. The effective pore size of the filter decreases with increasing depth.
2. Background Information
Depth filters have been available for a number of decades. U.S. Pat. No. 956,832 describes a filtering unit consisting of "intimately connected strata, each stratum being homogeneous in respect to its porosity and the different strata having pores of different degrees of fineness, the several strata being arranged transversely to the direction of the flow of liquid therethrough." The materials described as being useful are naturally occurring materials such as cellulose, asbestos fibers, and cotton fibers because synthetic fibers were unknown at the time.
More recent descriptions of depth filters have broadened the scope of fibers used in their preparation. Depth filters for blood filtration are described in U.S. Pat. No. 4,701,267, where multiple layers of nonwoven mats of increasing bulk density are used as filters. These mats can be made from synthetic fibers, semi-synthetic fibers, regenerated fibers, inorganic fibers and natural fibers, with polyester being exemplified. The nonwoven mats comprise fibers of an average diameter of from 0.3 to less than 3 .mu.m.
U.S. Pat. No. 3,353,682 describes a multilayer depth filter for biological filtrations, where the filter is made in a wet-laid fashion from short fibers. A typical construction includes a fine filter having pores in the range of 10 to 25 .mu.m and a course filter having pores in the range of 70 to 150 .mu.m. These filters are said to be capable of absolutely removing from fluids particles "as small as 25 microns in size, and even particles of 10 microns down to 0.03 microns and smaller." Above the fine filtering portion of the filter is a layer that protects the fine filter from being clogged by particles up to 150 .mu.m in size. Thus, the overall construction acts as a type of depth filter. In addition to natural fibers, synthetic fibers including polypropylene are said to be useful. Binders for the fibers are discussed in detail.
U.S. Pat. No. 4,925,572 describes a leukocyte filter for blood which uses gradient density filters made from nonwoven materials. One embodiment describes a series of 15, 10, and 7 .mu.m polybutyleneterephthalate (PBET) filters below a "gel filter" made from 30 to 50 .mu.m PBET. Modification of the fiber surface, by grafting, priming, or other "preconditioning means" is required to decrease the critical wetting surface tension of the filter mats.
U.S. Pat. No. 5,338,448 describes solid phase chromatographic column which employs a "guard disk" as a protective pad above the absorptive material of the column, retaining both particles and dissolved contaminants. The guard disk comprises sorptive chromatographic material incorporated into a woven or nonwoven fabric or membrane, or may comprise a fabric or membrane matrix having stationary phase functional groups covalently bonded to the fabric or membrane structure itself. The guard disk must contain sorptive material. The protective disk may comprise a plurality of fabric or membrane layers, and pore sizes for the protective disks are in the range of 0.1 to 5 .mu.m.